When it comes to our vast universe there are a great many terms and concepts to come to grips with. One such term is quasar. A common question we hear is “What’s a quasar?” In this article we will attempt to give you an answer to this question.
History of the Observation of the first Quasars
It became clear from work by Heber Curtis, Ernst Öpik and others between 1917 and 1922 that some objects seen by astronomers were in fact distant galaxies much like our own Milky Way. Things however became a little more confusing in the 1950s when astronomers noticed among the galaxies a small number of anomalous objects that defied explanation.
These anomalous objects were shown to emit radiation of many frequencies although for the most part no visual source for this could be located. Rarely however a faint point-like object similar to a star could be seen.
First Observed Quasars
It was in the 1950s that the first two quasars were identified (3C 38 and 3C 273) initially as radio sources in all-sky radio surveys. It took the use of the Lovell Telescope as an interferometer to determine these quasars had a small angular size.
It would take less than a decade for hundreds more of these phenomena to be mapped and recorded. In 1963 the radio source 3C 48 was eventually matched with an optical object. It appeared to be a faint blue star at the point at which the radio waves emanated.
How Did Quasars Get Their Name?
As with many things scientific early terms tend to be descriptive and a little long for common parlance. When scientists started to become aware of quasars they would refer to them as quasi-stellar radio sources.
The term quasar as a shortened form of quasi-stellar radio sources is attributed to an article written by astrophysicist Hong-Yee Chiu. Published in Physics Today in May of 1964 it was used as a term to describe these astronomically puzzling objects.
How Many Quasars have Been Discovered?
As of 2020 there had been over 700,000 quasars discovered by scientists with thousands more of various types being found each year. Based on observations these are somewhere in the magnitude of 600 million to 29.36 billion lightyears away from us.
Their vast distance from us means that we view them based on the light that has traveled to us, which has in some cases taken billions of years. So we see them as they were close to the beginning of the universe.
What Does a Quasar Look Like?
Visible and ultraviolet light emit from a glowing disk of material as it falls into a super massive black hole. Even hotter gases above the disk emit X-ray energies. At the black hole’s poles jets of gas emit everything from radio waves to X-rays. Prolific dust and gas glow at infrared wavelengths further away from the black hole as they move gradually toward the center.
Known as an accretion disk, the visible aspect of quasars are a few light-days across. This is miniscule in comparison to the size of a galaxy, for example the Milky Way is 100,000 light years across. However a quasar can outshine the galaxy in which they are found.
What Does Accretion Mean?
In basic terms accretion is the accumulation of particles into a more massive object through gravitational attraction. Essentially matter, typically gaseous matter, is drawn to a central point often creating a so-called accretion disc. This process is believed to have formed astronomical objects, such as galaxies, stars, and planets.
Essentially then the formation of a black hole which produces intense gravitational pull creates an accretion disc. This is an important aspect in understanding the nature of quasars.
How Do We See Quasars?
Although it takes powerful telescopes to see quasars it is still astonishing that we can see them from billions of light-years away. However when you consider just how bright these quasars are it becomes less surprising.
When we consider that the Milky Way, our own galaxy, is home to between 200 – 400 billion stars it may shock you to learn that a quasar can emit thousands of times the energy produced by all of your galaxy’s own stars.
So even though these quasars are so far away from us their luminance blocks out that of their own galaxy’s stars and is so intense that light from them reaches us albeit billions of years after the initial emission.
What Is a Quasar?
It took science some time to get to grips with what exactly quasars are but they can confidently now explain these powerful sources of light and radiation. Essentially quasars are caused by super massive black holes.
These vast black holes, some of which are billions of times larger than our own sun, draw in the gases surrounding them with powerful gravitational fields. Although a black hole as the name suggests does not emit light the friction caused by the extreme gravity pulling the gases beyond the event horizon causes superheating.
As gases and matter spiral toward the black hole the super heating emits radiation and light energy. The radiation spans the electromagnetic spectrum. In our closer neighbor galaxies there is less gas to be drawn in so we do not usually see quasars close to us.
The nearest quasar to earth is Markarian 231 which is roughly 600 million light-years from us. Although once the material drawn into the black hole passes the event horizon whilst still outside of this area the matter can flow due to superheating.
Light and radio waves can escape whilst still outside of the event horizon but once they enter the black hole itself all light and waves are trapped and can no longer escape. Essentially then even though we can not see the super massive black hole the accretion disc of light and radio waves outside the event horizon known as a quasar tells us that one exists.
Final Thoughts
Quasars are sources of intense light and radioactivity which are caused by the extreme gravitational pull of a super massive black hole. These phenomena in space are millions if not billions of light-years away from us but super heated dust and gases are detectable by our technology and have been since the early 1900s.